Tubular electric lamp



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United States Patent TUBULAR ELECTRIC LAlVIP Eugene Lemmers, ClevelandHeights, Ohio, assignor to General Electric Company, a corporation ofNew York 7 Application April9, 1956, Serial No. 577,017

25 Claims. (Cl. 313109) This invention relates to electric lamps andsimilar devices having elongated tubular envelopes. The instantapplication is a continuation-in-part'of my copending application,Serial No. 475,036, filed December 14, 1954, entitled ImpositionResistant Lamp Envelopes, assigned to the same assignee as the instantapplication, now abandoned.

Certain aspects of the invention relating to structural features ofvitreous envelopes are particularly useful in connection with lowpressure discharge lamps but may be used generally with electric devicescomprising a sealed elongated vitreous envelope. Other aspects of theinvention relate specifically to low pressure resonance radiationdischarge lamps wherein the radiation is emitted from the dischargemedium by diffusion, and make possible in such lamps higher loading orhigher efliciency simultaneously with highly advantageous directionaleifects in the radiant output.

In the case of fluorescent lamps of the usual low pressure positivecolumn type in which phosphors are excited by the resonance radiation ofmercury, as well as in other resonance radiation lamps (such asgermicidal mercury lamps, and sodium lamps), it is possible, as pointedout in my Patent-No. 2,482,42l-Flat-Tube Electrical Device, to improvethe radiation efliciency at a given loading by increasing the ratio ofperimeter to area of the cross section (henceforth abbreviated p./a.).Thus, increasing the p./ a. ratio permits either improved efliciency ata given wattage per unit length of lamp, or higher wattage loading perunit length for the same efliciency. For a given perimeter, a tube ofcircular section has the lowest p./a. ratio. The ratio may be increasedby departing from a circular section, for instance, by flattening thetube or otherwise deforming it out of round.

The 'manner in which increasing the p./a. ratio improves the radiationefliciency is not perfectly understood. According to one theory, theionizable medium including the mercury vapor, in reabsorbing resonanceradiation, reduces the proportion which reaches the phospher at theenvelope wall, thereby reducing the radiation efliciency.

:The degree of reabsorption will vary with the distance which the meanunit of radiation or quantum must travel before reaching the envelopewall; evidently, this effect will be less pronounced when the p./a.ratio is increased by deforming a round tube inasmuch as the envelopeperimeter is greater for a given cross sectional area of discharge, and,correspondingly, the mean distance to the envelope wall is therebyreduced. Another theory is that the electron temperature in theionizable medium is determined in part by ion losses to the envelopewalls. A greater envelope surface then produces a higher wall loss andthe electron velocity increases to maintain the discharge, with aresulting increase in efliciency of generation of resonance radiation.Probably the mechanisms proposed by both theories play a part in theincrease in efliciency with increase of the p./a. ratio.

Notwithstanding the advantages of fllat tubes over round tubes fordischarge lamps, flat tubes have presented 2,915,664 Patented Dec. 1,1959 a very serious drawback which has prevented their use to any greatextent: namely, weakness against implosion or inward collapse byexternal atmospheric pressure when they are evacuated. The gaseousfilling of low pressure discharge lamps, such as fluorescent lamps, isat such a. low pressure relative to the atmosphere that, for the presentpurpose, the envelopes may be considered evacuated. In my Patent2,482,42lFlat-Tube Electrical Device, there is disclosed a way ofimproving the resistance to collapse of flat-tube lamps which consistsin prestressing the narrow longitudinal edges of the flat tube to astate of permanent precompression strain. This may be effected byheating the tube above the strain point of the vitreous material andthen cooling the narrow edges of the tube before its wide approximatelyflat longitudinal walls cool below this temperature. Whereas suchtreatment does provide a definite improvement in implosion resistance,it does not lend itself to providing the configuration and form whichcharacterizes my present invention.

It will be appreciated that present day fluorescent lamps are acompromise between radiation efficiency which is increased by making thetube smaller and lowering the current density, and quantity of radiationor light produced which increases with the cross sectional area of thetube and the current density. Since a commercially acceptable lamp mustproduce a reasonable quantity of light, there are obviously practicallimits to increasing the efficiency by reducing the size of the tube;conversely, the requirement for efficiency has imposed limits on themaximum size of tube.

Accordingly, an object of the invention is to provide an elongatedvitreous tube of unique form or configuration having a perimeter-to-arearatio greater than round tubes of like cross sectional area and havinggreater strength and higher resistance to implosion than simpleflattened tubes.

A specific object is to provide an elongated low pressure discharge lampof unique form having a ratio of perimeter to area such as to permit asubstantial increase in loading without reduction in efliciency bycomparison with conventional round tube lamps, and which has sufiicientresistance to implosion by external atmospheric pressure as to make apracticable lamp Without requiring excessive thickness of the envelopewalls.

Another object of the invention is to provide an elongated low pressureresonance radiation discharge lamp having a discharge space'crosssection of improved form making the lamp capable of a higher loading fora given efi'iciency than heretofore possible and causing it to emit ahigher proportion of its radiation in a given sector of its crosssection.

A further more specific object of the invention is to provide animproved low pressure fluorescent discharge lamp utilizing resonanceradiation of mercury vapor and permitting a higher loading and lumenoutput per unit axial length at a given efficiency than heretoforepossible and at the same time providing a preferential light output in agiven sector of its cross section.

According to one aspect of the invention, a resonance radiation lamp ofgreatly increased loading capacity at a given efliciency and having thedesired radiation pattern simultaneously with improved implosionresistance, comprises an elongated tube of light-transmitting materialformed with a longitudinally extending transversely reentrant portion orgroove. The cross section of such a tube appears kidney-shaped, as seenin the drawings, and may be generally described as a flattened tubetransverselyrolled up into an inverted U-shape. The cross section of thedischarge space is in general a sector of an annulus defined bygenerallyconcentric walls and bounded by rounded convex edge walls.

In a preferred embodiment of the inventionwhich is particularlyadvantageous with respect to structural qualities of the vitreous tube,that is, resistance to implosion, the cross section of the tube, whileretaining the general shape of a sector of an annulus, is modifiedsomewhat to provide a re-entrant portion or groove having divergentinner side walls. The side walls merge inwardly of the tube into thecurvature of the inner concave wall of the groove and outwardly of thetube into the curvature of the outer edge walls which in turn join tothe convex outer concentric wall of minimum curvature (maximum radius).

In another embodiment of the invention, the longitudinally extendinggroove is provided in portions which are relatively short and spacedapart and alternate on opposite sides of the tube, giving a dimpled orcrenelated appearance. With this configuration, all re-entrant portionsof the envelope wall have a double curvature, that is, a curvaturetransverse to the axis of the tube and a curvature along the axis of thetube, thereby providing additional increase in strength and resistanceto implosion. This particular configuration consisting of short groovesections alternating on opposite sides has been found particularlyadvantageous in providing suitable implosion resistance to thin walltubing of large size.

According to another aspect of the invention, I have found that theprovision of a re-entrant portion in the envelope wall of a dischargedevice wherein the useful radiation is emitted from the discharge mediumby the process known as resonance radiation diffusion, produces totallyunexpected effects in connection with higher efiiciency andconcentration of light or radiation in the re-entrant portion. Thisaspect of the invention appears limited to discharge devices or lampsoperating with a plasma, that is, a region having substantially equalconcentrations of electrons and positive ions, wherein atoms are excitedto the resonant state and produce photons which reach the surface by thediffusion process. In this process, a photon or quantum of radiationwhich originates through the fall of an atom from an excited level toits ground level, is reabsorbed and re-emitted many times before itreaches the surface of the discharge medium and finally escapes. Thisrecurrent absorption is sometimes referred to as imprisonment ofresonance radiation. At each re-absorption, the photon raises an atom tothe excited level. The excited atom has a random movement in the plasmaand shortly falls back to the ground level whereupon a photon of lightis again emitted. This is the process of generation of the mercuryresonance radiation 2537 A. (ultraviolet) in the ordinary fluorescentlamp. It is likewise the process which occurs in the sodium lampproducing 58905896 A. (visible yellow) radiation.

In such lamps, that is, resonance radiation lamps a totally unforeseenresult of the instant cross section is that the plasma appears toconform itself more closely to the re-entrant portion, or statedotherwise, the plasma hugs the groove, and the groove or re-entrantportion receives resonance radiation per unit area at a rate muchgreater than the mean of the envelope per unit area. The resonanceradiation is converted by the phosphor which coats the envelope wallsinto visible radiation or light. The net result is that light is emittedfrom the region of the groove at a high efiiciency and the groove has amuch higher brightness than the lamp as a whole. Actual measurementshave shown the groove to be as much as 40% brighter than the envelopewall on the opposite side. This increased brightness of the groove maybe used to achieve directional effects which are high ly advantageous inlighting applications. The remarka ble and totally unexpected brightnessof the groove is an outstanding feature of the invention.

For a more detailed description of the invention, attention is nowdirected to the following description and. accompanying drawings. Thefeatures of the invention:

4 believed to be novel will be more particularly pointed out in theappended claims.

In the drawings:

Fig. 1 is a pictorial view of a discharge lamp embodying the inventionwith a continuous transversely re-entrant portion or groove extendinglongitudinally along its underside, portions of the envelope wall beingbroken out to shorten the figure.

Fig. 2 is a similar view of another lamp. embodying the invention withshort lengths of groove or indentations alternating on opposite sides.

Figs. 3, 4, and 5 are side elevation, longitudinal section and crosssection views, respectively, of the lamp of Fig. 2. In the case of Fig.5, the view represents also a cross section of the lamp of Fig. 1;

Fig. 6 is an enlarged cross-sectional view of an improved re-entrantgroove configuration which has been found particularly advantageous inrespect of structura qualities, that is increased implosion resistance.

Fig. 7 is a polar diagram of the light distribution achieved with are-entrant groove lamp in accordance with the invention such asillustrated generally in Fig. 1.

Fig. 8 is a graph affording a comparison of the loading capacity ofre-entrant groove lamps with prior lamps.

Fig. 9 is a diagram illustrating the temperature distribution about thecross section of re-entrant groove lamps at different loadings.

Fig. 10 is a diagram illustrating the etfect of the boundaries upon theradiation diffusion process.

In order to facilitate the detailed description of the invention, itwill be presented under the following headings:

(l) The Grooved Lamp (2) Improved Grooved Lamp Cross Section (3) GroovedLamp Design Considerations (4) Light Distribution (5) Factors ofEfiiciency (6) Theoretical Considerations (7) Reduction in ElasticLosses (8) Temperature Distribution (9) Vapor Pressure RegulatingProperty (10) Phosphor Lumen Maintenance (11) Physical Basis of GrooveBrightness (12) Crenelated Grooved Lamp (13) Implosion Resistance (14)Manufacture 1) The grooved lamp Referring to Fig. 1, there is shown afluorescent lamp ll of the low pressure, positive column type embodyingthe invention. The lamp comprises an elongated envelope 2 havingcircular or round tube ends 3, 3 which are annularly reduced orshouldered at their extremities for securing thereto bases 4, 4, eachprovided with a pair of insulated contact terminals or pins 5, 5 and 6,6. As shown at one end of the lamp, the electrode mount or stem flare 7is sealed peripherally into the circular tube end and includes a press 8through which are sealed current inlead wires 9, 11. The inwardprojections of the lead wires support the filamentary cathode 12,whereas the outward projections are connected to the terminal pins 5, 6.Cathode 12 may consist of a coiled-coil of tungsten wire provided withan over-wind and coated with an activated mixture of alkaline-earthoxides, such as the usual mixture comprising barium and strontiumoxides. The other end of the lamp is provided with a similar cathode andone of the stem flares is provided with an exhaust tube which is sealedor tipped off in the usual fashion. The cathodes may be of the lowresistance and low thermal capacity rapid start type disclosed in mycopending application Serial No. 250,106, filed October 6, 1951, nowPatent No. 2,774,918, Electric Discharge Device, and assigned to thesame assignee as the present invention.

The lamp contains an ionizable atmosphere including a starting gas ormixture of one or more of the inert rare gases-of group of the periodictable-at a low pressure, forinstance argon at a pressure of 0.5 tomillimeters of mercury, and mercury-vapor. The droplets of mercuryindicated at 13 and 13 exceed in amount the quantity vaporized duringthe operation of the lamp wherein themercury vapor exerts apartialpressure in the range of 1 to 20 microns for optimum generation ofmercury resonance radiation at 2537 A. The exact value of partialpressure of mercury vapor for optimum generation of 2537 A., that is formaximum lumens, may vary with the kind of starting gas, that is, whetherargon, krypton or xenon, with the pressure of the starting gas, and withthe current density. Where-the starting gas is argon at approximately 3millimeters pressure, the optimum pressure of mercury vapor is usuallyfrom 5 to 8 microns.

The phosphor coating indicated at 14 on the inside of the envelopeconverts the 2537 A. resonance radiation into visible light. It may be ahalophosphate phosphor activated with antimony and manganese as per U.S.Patent 2,488,733, McKeag et al. of the assignee as the presentinvention, and producing a cool white light. The envelopemay be coatedexternally with a water-repellent or hydrophobic coating to facilitatestarting under high humidity or adverse atmospheric conditions. Asuitable coating is a hydrolized organo-silicon halide as described inU.S. Patent 2,408,822, Tanis of the assignee as the present invention.'It may be produced by exposing the lamp for a few minutes to the vaporof methylchlorosilane for instance, in a suitable enclosure maintainedat 50% relative humidity;

In accordance with my invention, vitreous envelope 2 is provided with atransversely re-entrant portion or groove 15 extending longitudinallysubstantially throughout its length between the round ends 3, 3. Theenvelope, as shown in Fig. 5, may. be generally described as a flattenedtube which has been rolled up transversely into an inverted .U-shape.More exactly, the cross section of the discharge space may be regardedasa sector of an an nulus defined by generally concentric walls 16 and17 and bounded by rounded. convex edge walls 13, 18. Convex outer wall16 has the minimum curvature, its radius being that of the originalround tube from which the instant grooved tube was formed. Concave innerwall 17 has a greater average curvature than outer wall 16, its radiusof curvature being approximately one-third that of outer wall 16. Theconvex joining or edge walls 18, 18' would in theory, assuming perfectlyconcentric inner and outer walls anda mean radius of curvature for innerwall 17 exactly one-third that of the outer wall 16, have exactly themean radius of curvature of concave inner wall 17. In practice, however,convex edge walls 18, 18' are provided with a slightly greater meancurvature than concave inner wall 17. This is done because whereas it isdesirable to have the wall-to-wall spacing substantially constant, it isessential to avoid a constriction at the center. If such a constrictionwere permitted, the discharge would not fill the discharge spaceuniformly and would tend to occupy the space to one side or the other ofthe constriction. Since the molding of glass cannot in any event beperformed with perfect accuracy, a practical soluction resides in makingthe radius of curvature of the convex edge walls somewhat less than thatof concave inner wall 17, or somewhat less than one-half the wall-towallspacing of the concentric inner and outer walls 17, 16.

A number of lamps like that illustrated in Figs. 1 and 5 have been madeby suitably reshaping the envelopes or tubes used for the l00-watt,60-inch long fluorescent lamps'commonly designated F100Tl7. These tubesare approximately 2% inches in outer diameter (17 units of A; inch) andconsist of lime glass with a nominal wall thickness of .050 inch. Such atube can be flattened to a cross section of approximately 1 by 3 inchesin dimension. Thus flattened, the tube can hardly withstand 1atmosphere, that'is, a pressure of 15 pounds per square inch,

"6 without imploding. However, when thetubeis formed to an inverted-U-shapedsection as illustrated in Fig. 1, it canwithstand about 2atmospheres before implosion. Thus, the longitudinally extendingre-entrant groove configuration in accordance with the inventionhasatleast doubled the implosion resistance over that of a flattenedtube of equivalent cross section.

Experience has taught that forsafety in handling and long-term usage,lamp envelopes should have an implosion resistance of five atmospheresor more, round tubes being in this range. In order to increase theimplosion resistance of the lamp illustrated in Fig. 1 to 5 atmosphereswhen made with T17 tubing, the tube wall must be made thicker, ashereinafter disclosed by way of example and discussed under thenextheading. Alternatively, with the same wall thicknesspreviously'm'entioned, namely .050 inch, the-desired'degree ofimplosion'resistance could be achieved by reducing the cross-sectionaldimensions of the tube.

(2) Improved 'groov'ed lampcross section' A preferred embodiment of theform of my invention illustrated in Fig. 1 having a continuous groove onthe underside of the tube has the cross section illustrated in Fig. 6.This cross section has been found particularly advantageous with respectto resistance'to'implosion.

Referring to Fig. 6,- the cross section retains the general shape of asector'of an annulus with an outerconvex wall 16 of minimum curvatureand concentric concave inner wall 17 of greater curvature. The convexjoining or edge walls 18, 18 have a curvature slightly greater than thatof-concave inner Wall 17. The curvatures'cliscussed herein are those ofthe inside surface of the glass which bounds the discharge space.However, the present cross section is modified from that disclosed in myprior copending application No. 475,036 Implosion Resistant LampEnvelopes and illustrated in Fig. 5 of this application, in that thegroove is provided with more or less straight slanting sections 19, 19interposed between the curvatures of the top of the groove and of theedge walls. The side walls of the groove are thus outwardly divergent,that is, slanted downwardly and outwardly. Stated otherwise, the sidewalls of the grooveare slanted like an inverted V rather than aninverted U and vertical side wall sections, such as appear in Fig. 5.at19a, 19'a with reversals of curvature on each side, are avoided."

Grooved test lamps made -with'the cross section illustrated in Fig. 6have shown an increase in implosion resistance of as much as 30 percentover that of similar test lamps made with the cross section shown inFig. 5. Analysis of these test results indicatesdesirable dimensions inlamps made with T17 size tubing of nominal outer diameter 2% inches witha wall thickness of approximately .075 inch to be as follows. The radiusof curvature T of outer wall 16 is 1.0625 inches measured to the outersurface. The radius of curvature A at 17 in the top of the groove is0.3125 inch measured to the outer surface of the glass. The radius ofcurvature B of the edge walls 18, 18 is 0.3125 inch measured again tothe outer surface of the glass. The center of the radii of curvature ofthe edge walls is located adistance C equal to 0.0938 inch below thecenter of the radius of curvature of the concave inner wall of thegroove. The slanting side walls of the groove slope outwardly at anangle 0 to the vertical which should be at least 15 in order to realizesubstantial benefits: in the illustrated cross section, 0 is 27.

It will be appreciated that whereas the radii A' and B of the groove andof the edge walls are equal as measured, when viewed from the inside ofthe envelope the radius of the edge wall is actually less than that ofthe groove by twice the thickness of the glass.' In other words, taking.3125 inch as the measured radius to the external glass surface in eachcase, and taking .075 inch as the thickness of the glass, the effectiveradius of curva- 7 ture of the groove as defining the discharge space is.3875 inch and that of the edge walls is .2375 inch. The radius ofcurvature of the edge walls as seen from the discharge space is thusless by approximately 39% than that of the groove, likewise as seen fromthe discharge space. These lamps show an implosion resistance ofapproximately 90 lbs. per square inch when tested under compressed air.The illustrated cross section thus is eminently satisfactory from thepoint of view of implosion resistance and in fact surpasses theimplosion resistance safety requirement of atmospheres. The foregoingspecific lamp dimensions have been given by way of example only, andobviously the principles involved can be applied to other lamp sizes.

(3) Grooved lamp design considerations In designing a re-entrant groovelamp, several factors must be taken into account in determining theenvelope geometry or cross section. The following mathematicalexpressions state approximately the changing characteristics of thedischarge in terms of its geometry:

Vg=lc a may be referred to as the figure of merit for loading. Ingeneral, the watts per foot to which a fluorescent lamp may be loadedwithout dropping below a given efliciency in lumens per watt will varyproportionally with the figure of merit. As a round tube is deformed outof round, as by forming a re-entrant groove, the figure of meritinitially increases very slowly, but then more and more rapidly as thedepth of groove increases. For the purposes of this discussion, it isconvenient to refer to the degree of equivalent flattening by analogy tothe simple flattened tube discussed in my Patent 2,482,421. The degreeof equivalent flattening may be taken as the ratio of curved annularbreadth of the discharge space, given by D in Fig. 6, to the maximumwall-towall spacing opposite the groove and given by E.

Equivalent flattening in a ratio of 2:1 is about the least to offer anyreal advantage. Any lesser degree of equivalent flattening has suchslight eflect on the figure of merit as not to be worthwhile. Higherdegrees of equivalent flattening, for instance 3:1 and up, are verydecidedly advantageous.

The upper limit to the degree of equivalent flattening may be set by thecharacteristics of the discharge or by the structural limitations of thevitreous envelope and the necessity for avoiding sharp bends whereexcessive strai may develop in the glass.

The pertinent electrical characteristic of the discharge is the tendencyto constrict, that is, the tendency of the diffuse positive column todraw away from the narrow edge walls and assume a more generallycylindrical cross section. This tendency appears to be connected withthe phenomenon of two stage ionization which is notably present in adischarge medium consisting of mercury vapor and an inert starting gas.The tendency to constrict may be reduced by reducing the inert gaspressure, for instance to less than 1 millimeter. With a starting gasconsisting of argon at 0.5 millimeter pressure, equivalent flattening ina ratio of 6:1 is possible without excessive constriction of the plasma.Thus the cross section illustrated in Fig. 6 applied to T17 tubing givesD equal to 2.972 inches and E equal to 0.600 inch and provides anequivalent flattening of approximately 5:1; such lamps operate with theplasma substantially filling the cross section. Increase in the degreeof equivalent flattening be yond 6:1 results in greater constriction ofthe discharge and a limit is reached, for instance at approximately 10:1beyond which further equivalent flattening is ineffective, because theplasma will not spread out to the edges of the discharge chamber.

Closely related to the degree of equivalent flattening is the degree oftaper of the wall-to-wall spacing toward the edge walls 18, 18', thatis, towards the ends of the depending leg portions. As has previouslybeen mentioned, the discharge space cross section theoretically shouldbe a sector of an annulus. However, in practice, some taper ispreferable in order to counter manufacturing variables and assurestability of the discharge. In the cross section illustrated in Fig. 6,the maximum wall-to-wall spacing over the groove is given by E equal to0.600 inch and the minimum wall-to-wall spacing at the beginning of theedge wall curvature is given by F equal approximately to 0.475 inch.Thus, the taper is approximately or 21% and this degree of taper assuresstability simultaneously with substantial filling of the cross sectionby the plasma. With high degrees of taper, for instance, degrees oftaper exceeding 50%, constrictive eflFects become pronounced and theplasma will not penetrate into the leg portions to the edge walls. Ofcourse, very highly tapered cross sections, such as a lune or crescentare decidedly to be avoided because the plasma will not spread into thesharp recesses.

It is to be noted that the degree of equivalent flattening in adischarge cross section in the shape of a sector of an annulus having anopen groove is immediately related to the breadth of the re-entrantgroove 17 and to the depth to which the groove re-enters the circularouter Wall 16. Thus, when the degree of equivalent flattening isspecified, the cross section is generally determined; othercharacteristics, such as the degree of taper across the annular breadthof the discharge cross section and the angle of divergence of the sidewalls of the groove may then be determined according to theconsiderations which have been discussed above.

In resonance radiation lamps Where secondary ionization effects are lesspronounced or possibly substantially absent, as in sodium vapor lamps,theoretical considera tions indicate that the tendency of the plasma toconstrict should be much less. Accordingly in such lamps, the limits asto the permissible degree of equivalent flattening may exceedconsiderably those which have been given above for mercury resonanceradiation lamps.

(4) Light distribution The most striking feature of a resonanceradiation lamp with a longitudinally extending re-entrant portion orgroove is the remarkable brightness of the groove. This result istotally unexpected and none of the prior Work done with flattened tubingor other non-circular shapes gave any indication of the possibility ofthe instant development. The high brightness in the groove produces anasymmetrical distribution of light, which, in

its flux below the horizontal and 41.4% above.

ass-15,654

,lighting applications, can result in an increase of as much as 50percent in light on the working surface. The increase considered here isdue entirely to the asymmetrical distribution and is in addition to theother properties of higher efficiency or loading capacity of there-entrant groove lamp.

Referring to Fig. 7, there is shown a polar diagram of the lightdistribution achieved with a re-entrant groove lamp in accordance withthe invention. The cross section of lamp 1 is reproduced at the centerof the polar diagram and the radial distance from the center to anypoint on curve 21 indicates the candlepower in the direction which thepoint makes with the origin. Integration of the candlepower curve over agiven sector gives the total luminous output or fiux of the lamp in thatsector. When this is done, it is found that the lamp delivers 58.6% ofFurthermore, the lamp delivers 38% of its flux in the 90 sector orquadrant which includes the groove, that is the quadrant including 45 oneach side of a vertical line down from the center of the groove. It willbe appreciated that with a round lamp having a symmetrical lightdistribu- -tion, the light flux in any quadrant is 25% of the total, so

that the instant re-entrant groove configuration provides an increase ofapproximately 50% in light output in the lowermost quadrant.

The flux distribution pattern illustrated is highly advantageous forgeneral illumination applications wherein it is usually desirable toincrease the downward light component. With ordinary round tube lampshaving a circular flux distribution, this must be achieved by therefiectingsurfaces of the fixture. The fixture will introduce losseswhich are aggravated by dust and dirt on the fixture itself and on theupper surface of the lamp Where dust readily collects. It is alsofrequently desirable to reduce the light component at low anglesrelative to the horizontalin order to cut down glare. Fixtures willordinarily .do this by means of translucent side panels which becomeadditional sources of losses. The re-entrant groove flux distributionthus approaches of itself the ideal for many lighting applications,inasmuch as it has a reinforced downward component and reduced lateralcomponent without any of the losses which would be incurred in achievingsuch redistribution by means of the fixture.

(5) Factors of efiiciency The longitudinally extending re-entrant grooveconfiguration in a resonance radiation discharge lamp provides aconsiderable increase in efficiency at a given loading. Even moreremarkable, however, is the fact that it provides a tremendous increasein loading capacity for a given efiiciency.

Referring to Fig. 8, curves 22 and 23 illustrate respectivelytheluminous output of round and re-entrant groove lamps under similarconditions. Both were of 5 feet nominal length with equal perimeters,having been formed from T17 tubing. It will be observed that curve 22for theround lamp rapidly flattens out when the loading is increasedbeyond the conventional 20 watts per foot, whereas curve 23 for there-entrant groove lamp shows that the loading may be increased up to 35watts per foot before encountering a similar degree of flattening.

Comparative tests of re-entrant groove and conventional round tubefluorescent lamps in the T17 size have shown that at loadings of 20watts per foot, their efl"1- ciencies are approximately 59 and 48 lumensper watt, respectively, the re-entrant groove lamp being approximately23% more efficient. Even more remarkable, however, is the fact that there-entrant groove lamp can be loaded up to 36 watts per foot before itsefficiency drops to the level of the standard T17 round lamp at itsconventional 20 watt per foot loading, that is, 48 lumens per watt;thus, for the same efficiency in lumens per watt, the re-entrant grooveconfiguration permits an increase of approximately 80% in loading.

. invention.

The luminous efiiciency figures considered above 'were determined usinga halophosphate phosphor producing-a standard cool white (4500 K) colorwith which the light conversion efficiency is nearly equal to thatobtained with a standard warm White (3500 K) color, being but a fewpercent (2% to 4%) lower. As is well known, in the case of the standardwarm white (3500 K) color, the average luminosity is approximately 47%of the luminosity of the 5540 A. yellow-green line to which the eye ismost sensitive, and the conversion by the phosphor from the 25 37 A.ultraviolet line proceeds according to a quantum ratio of approximately44% at a utilization efficiency of approximately 86% lowering thestarting gas pressure into the range of 0.1

to 1.0 millimeters of mercury as taught in my copending application No.475,035, filed December 14, 1954,

now abandoned, entitled Low Pressure Discharge Lamps, and assigned tothe same assignee as the present For instance, by using for the startinggas argon at 0.5 millimeter pressure, it is possible to achieve loadingsin excess of 40 watts per foot at an efiiciency at 48 lumens per watt,thus more than doubling the loading capacity over the round tube lamp atthe same effi- 'ciency.

:lumens per watt; for a cool white color (4500 K), the

efiiciency may be a few percent lower. The wall loading in prior artlamps having acceptable efficiencies was generally under 0.043 watt percm. (0.28 watt per in. a much used figure being 0.02 watt per cm. (0.13watts .per in. maximum linear loading was approximately 20 watts perfoot, the most generally used figure being -10 watts per foot. Handbook,copyright 1947 by the Illuminating Engineer- (See for instance I.E.S.Lighting ing Society, pages 6-40, Figs. 6-36.) The re-entrant groovecross section makes practical wall loadings up to approximately 0.08watt per cm. the most useful range being approximately 0.05 to 0.08watts per cm. (0.32 to 0.52 watts per in. and linear loadings up toapproximately 40 watts per foot. Thus, for instance a T17 reentrantgroove lamp having a perimeter of 16.9 cm. (6.66 in.) and a crosssectional area of 12.1 cm. (1.88 in?) operating at 2.5 amperes arccurrent with a voltage drop of approximately 84 volts in a 5 footlength, consumes approximately watts for a light output of 10,000 lumensat an efficiency of 57 lumens per watt, a wall loading of 0.07 watt percm. a linear loading of 35 watts per foot and a current density of 0.2ampere per cm. In practical laymans language, the foregoing exampledemon- "strates that the invention permits the same physical size oflamp to produce twice as much light without any loss in efficiency, infact with some gain.

It will be appreciated that the above ranges of parameters represent aradical departure from prior limits and constitute an outstanding stepforward in the art.

(6) Theoretical considerations 'a smallerchance. of destruction byadissipative collision.

The second answer is based on thetheory that the elecilll trontemperature or velocity in the ionizable medium is determined by lossesto the envelope walls. An increase in the Wall perimeter without acommensurate increase in the discharge cross section then produces ahigher wall loss, so that the electron velocity increases to maintainthe discharge, thereby enabling more efiective production of 2537 A.resonance radiation.

(7) Reduction in elastic losses fluorescent lamp, the elastic losses areapproximately of the total wattage consumption. When an electron strikesan atom and bounces off elastically, that is without excitation orionization of the atom, a small average fraction 1 of its energy isimparted to the atom, given by:

where m is the mass of the electron and M that of the atom. While thisfraction is exceedingly small, the rate of collisions with gas atoms influorescent lamps is so large that a considerable heating of the gasresults. Now the energy of the electron being proportional to electrontemperature T and the speed of the electron to VT; the energy R lost inthis way per second per electron will be given by:

Theoretical considerations show that the number of electrons variesapproximately inversely as the perimeter for a given cross sectionalarea of discharge. On the other hand, the electron temperature increasesbut at a much slower rate than according to the ratio of perimeters.Therefore, whereas an increase in perimeter for a given cross sectionwill result in an increase in the total elastic loss per electroncollision with an atom, this will be more than ofiset by theproportionally greater reduction in the total number of such collisions.The final result is a net reduction in elastic losses in the tube.

(8) Temperature distribution Another feature of the re-entrant grooveconfiguration, and which is believed to be an important factor in theremarkable increase in loading capacity without commensurate reductionin efficiency of the lamp, is the distribution of temperature about thecross section of the lamp when it is operated with the groove lowermost.This is the usual mode of operation inasmuch as the asymmetrical lightdistribution providing a greater intensity in the groove quadrant isnormally utilized with that quadrant facing downward.

Actual measurements of surface temperature of T17 re-entrant groovelamps operated at 26 and watts per foot in an ambient temperature of 24C. have shown the temperature distribution given by the two columns oftabulated figures in Fig. 9. Referring to the left-hand column, it isseen that the hottest point in the cross section is the top of thegroove which attains a emperaure of 65 C. at a lamp loading of 26 wattsper foot, while the coolest point is the bottom of the legs or joiningedge walls at a temperature of 36.5 C. Referring now to the right-handcolumn, it is seen that increasing the wattage to 35 watts per footraises the temperaure at the top of the groove to 77.5 C., but hardlyaffects the temperature at the bottom of the legs or edge walls whichremain at 36.5 C. All other points show an intermediate rise intemperature.

While the foregoing temperature measurements are fairly accuraterelative to one another, they are only approximate as to absolute valuessince they were obtained through thermocouple readings. Moreover exactvalues in any given case will vary of course with the ambienttemperature, air movement, and location of the lamp relative to othersurfaces. In the same vein, a somewhat higher temperature is to beexpected with the lamp mounted in a fixture, and such factors must betaken into account in designing the lamp for optimum vapor pressure inits normal environment.

Some effects of the temperature distribution described above will bediscussed hereafter under the next two headings to follow.

(9) Vapor pressure regulating property The foregoing temperaturedistribution demonstrates several factors which are pertinent toefficiency and to lumen maintenance. Firstly, the joining edge walls orlegs are the coolest part of the lamp and their temperature hardlychanges with increase in loading. This effeet is believed due toincrease in convection air current with increase in wattage dissipation,simultaneously with some constriction of the plasma within the lamp asthe loading is increased. The constriction of the plasma or dischargecauses it to draw away from the edge walls or legs of the cross sectionwith consequent reduction in heating effect in those regions. The twofactors operating together tend to maintain the temperature of the edgewalls substantially constant despite increase in loading, and indicatethat ambient air movement will have less effect on vapor pressure thanwith round tube lamps. Since the mercury vapor pressure is determined bythe coolest part of the lamp (36.5 C.), it is apparent that the mercuryvapor pressure will remain substantially constant at the design optimumdespite the increase in loading. The optimum mercury vapor pressure mayvary, of course, with different lamps, depending on fac tors such as thekind of starting gas, its pressure, and the current density.

Thus another unexpected and highly desirable property of the instantre-entrant groove configuration is the inherent or built-in mercuryvapor pressure regulating property. It is well known, of course, thatdeparture of mercury vapor pressure from the optimum entails a reductionin efficiency. Thus the factor of variability of mercury vapor pressure,which factor is degenerative of efficiency, is substantially reduced oravoided with the reentrant groove cross section, and this may serve toexplain in part the tremendous loading capacity of re-entrant groovelamps.

(10) Phosphor lumen maintenance The temperature distribution achievedwith the reentrant groove cross section is believed to be moreoverconductive to superior lumen maintenance. One factor of depreciation offluorescent lamps during life is connected with condensation of mercuryvapor on the phosphor coating with resulting decrease in efliciency ofconversion of 2537 A. radiation into visible light. The mercury vaporevidently will condense in the coolest part of the cross section of thetube. In the instant case, it is observed that the coolest part of thecross section is the depending legs or edge walls and the mercury vaporwill naturally condense in those sections. Concurrently, the minimumcondensation of mercury vapor is to be expected at the hottest part ofthe lamp, which is the top of the groove. As has already been pointedout, the top of the groove is the portion of the lamp that runs thebrightest and which is the most effective in producing light. Thebottoms of the legs or edge walls are the least effective parts of thelamp in producing light and may have a comparative brightness 30% ofthat of the top of the groove. Obviously, since condensation of mercurymust occur somewhere in the lamp, such con- 13 densation will: be theleast damaging in respect of reduction of efficiency if it occurs at thebottoms of the legs or joining edges, as indicated by the observedtemperature distribution. Thus another unexpected benefit ofthere-entrant groove configuratiin is the present factor of superiorlumen maintenance.

(11).. Physical basis of groove brightness The brightness of the groovein the remnant groove configuration is one of the most striking featuresof the invention. This feature is believed to result from the fact thatthe radiation produced in the plasma is emitted by the process ofdiffusion. It is only with resonance radiation lamps that this processoccurs to any substantial extent.

The resonance radiation of an atom is the spectral line .or linesproduced. by the fall of the atom from the first excited state or levelto the ground level. Lines of this kind are called resonance lines, andsince they are the most easily produced they represent the greatestproportion of all the transitions taking place at any one instantand arethe most intense of the-spectral lines. In mercury, theresonance'radiation line is 2537 A. and represents the drop in potentialof the mercury atom from the first excited level at 4.86.electron volts.In sodium, on the other hand, the first two excited levels are at 2.09and 2.1 electron volts and the falls from these levels to the-groundlevel produce 5896 and 5890 A. visible yellow radiation. Since the finallevel of the transition in the case of resonance radiationis the normalstate of the atom, the reverse process, namely the excitation of anormal atom by absorption of quanta of resonance radiation, is not onlypossible, but in view of the .high population of atoms in the plasma, ishighlylikely. It has been estimated that the number of re-absorptions ofa quantum of radiation or photon produced in a low pressure fluorescent.lamp in a T12 envelope (1 /2 inches in diameter) is in the range of 100.In this respect, the'ernission of radiation from a resonance radiationlamp is totally different from the emission in the case of anonresonance radiation lamp, such as a neon tube. In neon, forinstance,i6,402 A. visible red radiation is produced by the transitionof the atom from the 18.5 to the 16.6 electron volt level. In this case,of course, re-absorption of the quantum by raising an atom at the 16.6electron voltlevel to the 18.5 electron volt level is possible, but ishighly improbable in view of the =relatively very small population ofatoms at the 1616 level. photon of light is emitted from the plasmadirectly and substantially without any reabsorption.

The diffusion of the-photon in the case of resonance radiation proceedsaccording to the principle of the random walk. According to thisprinciple, N advances or' steps in perfectly random directions will onthe average'result in a progression to a point /N steps away from thestarting point. Considering the diffusion process taking place in adischarge space bounded by two cylinders'of radii R and RT, as shown inFig. 10, it is possible to arrive at certain conclusions with regardto'theion or electron density in the plasma within such a dischargespace. From elementary principles, the ion density n must be zero atboth surfaces. There will me maximum in ion density n at some radialdistance R as represented by the dotted circle. Inside R ions willdiffuse to the inner cylinder and outside R the will diffuse to theouter cylinder.

Considering the principle of the random walk and the fact that ions arerushing about in all directions, it is reasonable to expect that ionswill tend to congregate most densely in the region where on the wholethey are farthest away from the surfaces which bring about theirdestruction. Referring to Fig. and considering circle Q therein, thiscircle has its center q midway between the two boundary cylinders. Letthis circle be :deemed The'result is that in the neon lamp, the

a rees;

' 14 to represent the distance .of random diffusion of ions fromnitscenter after .a given time if there were no boundaries. .Now it is clearupon inspection that the outer concentric cylinder R has a greatersurface area :(chord .r) withincircle Q than does the inner concentriccylinder R' (chord r). Sincethe areas of the cylinders which encroachupon the circle are to some extent indicative of the ion. destructiveeffect of the boundaries, it follows, that the outer cylinder is moreeffective inabsorbing ions originating from the center of the circlethan is the inner cylinder. Accordingly, it is to be expected that Rwill be nearer to the inner cylinder than to the outer, .that is:

I lewd- 5i According to the above picturization, it is to be expectedthat the plasma will hug the groove and laboratory observations haveshown this to be the case. The rate of generation of quanta will, ingeneral, be proportional everywhere. to the concentration of electrons(and ions). Sinceall are moving by diffusion, the flux of resonancequanta to the boundaries willfollow the same pattern as the flux ofelectrons and ions. Therefore, quanta or photons generated on the innerside of circle R will diffuse to the inner cylinder R, whereas thosegenerated outside R will diffuse to the outer cylinder R. Theoreticalconsiderations indicate that the generation. of quanta on either side ofR is such, relative to the area of the inner and outer cylinders, thatthe inner cylinder R will receive radiation at a substantially greaterrate per unit area than the outer cylinder R. Inasmuchas the re-entrantgroove cross section is a sector of an annulus, the same generalconclusions will apply andare confirmed by the laboratorydeterminationthatthe brightness of the groove is 40% brighter than the envelope .wallonthe opposite side. Taking into account the internal transmission oflight (visible radiation, not 2537 A. resonance radiation) from theinsidessurface. of the groove to the envelope wall on the opposite sideand which operatesto reduce somewhat the relative brightness of thegroove, the above 40% figure is in substantial agreement with thatpredicted by more rigorous mathematical treatment of the problem. Inorder additionally to confirm the foregoing theory, a re-entrant groovelamp envelope with the cross section shown in Fig. 6 was filled withneon at a few millimeters pressure (about 3) instead of mercury and astarting gas. As previously mentioned, the visible radiation from neon.is non-resonance radiation so that there is substantially no diffusionof this radiation caused by multiple absorption andre-emission, suchradiation being emitted directly. Except at low current densities, thereis very pronounced constriction; for instance, at 1.5 amperes, a

comparatively low figure for a re-entrant groove lamp accordlng to theinvention, the arc constricts markedly to the center and will not spreadinto the dependent portions or legs bounded by the edge walls. At lowcurrents, for instance under milliamperes, the discharge begins tospread; nevertheless, the groove becomes no brighter. Except for slightlens effects, the radiation pattern, as determined by photocellmeasurments, is circular and substantially uniform in all directions atright angles to the longitudinal axis of the lamp, thus providingstriking confirmation of the fact that the benefits of the presentaspects of the invention can only be had with resonance radiation lamps.

Further confirmation of the foregoing theory is provided by the factthat the visible mercury lines (nonresonance) in a re-entrant groovelamp show no appreciable concentration in the groove. Thus if thephosphor coating is omitted from the lamp of Fig. 1 so that only thepale blue light from the visible mercury lines are ,di1:e c t Qn.S ..atrightangles to the axis of the lamp and l the groove appears nobrighter. On the other hand if the lamp envelope is made of anultra-violet transmitting glass as for a germicidal lamp and the 2537 A.resonance radiation pattern is measured with a suitable instrument, theconcentration of 2537 A. radiation in the groove sector is again noted.

Whereas it would appear that a perfectly annular dis charge spacedefined by concentric tubes would offer the advantage of high brightnessof the phosphor surface of the inner tube, it does not realize thedesired polar dis tribution including a reinforced downward light cornponent, nor does it offer any solution to the problem of eificientlyutilizing the light generated at the inner tube. Also a longitudinallyfluted tube, that is a tube with several grooves extendinglongitudinally of the axis, fails to realize the advantage of there-entrant groove cross section inasmuch as the plasma will spread onlyif the grooves are shallow; if they are deep, the plasma will constrictinto the center in either case, the advantages of high brightness of thegroove and inherent vapor pressure regulation will be substantiallyreduced or lost, and of course the desired polar light distributioncould not in any event be achieved.

(12) crenelated grooved lamp Figs. 2 and 3 to 5 illustrate another lamp31 embodying the invention and having yet greater resistance toimplosion than equivalent uniformly grooved lamps. Here the envelope 32is provided with spaced indentation or re-entrant portions 33, 34 ondiametrically opposite sides giving a dimpled or crenelated appearance.The indentations 33, 34 may be considered to be short sections of alongitudinal groove alternating on opposite sides of the envelope. Across section of the envelope through one of the indentations is similarto that of lamp 1 and is illustrated in Fig. 5. It comprises a con vexouter wall 16, a concave inner wall 17, and convex edges 18, 18'. Thegroove sections or indentations 33, 34 are relatively short. For maximumstrength of the envelope, the groove sections are preferably of a lengthnot in excess of three tube diameters, that is, not in excess of threetimes the diameter of convex outer wall 16: for instance, in a T17envelope of 2% inch diameter, the indentations may be 3-4 inches long.The groove sections may of course be made longer, but at the expense ofsome reduction in implosion resistance. Lamp 31 is provided, as is lamp1, with circular tube ends 3, 3 to which are attached the usual bases 4,4. In all other respects, lamp 31 may be similar to lamp 1 and includingsimilar electrodes and filling within the envelope.

A number of lamps 31 made from T17 tubing of .040 to .050 inch wallthickness have passed 70 pounds per square inch pressure tests, and havesince been standing at atmospheric pressure without implosion for aperiod of time well in excess of two years as of the filing date of thisapplication. Thus, the interrupted groove or crenelated configuration ofenvelope 32 provides a discharge lamp having an implosion resistancewell within the requirement for a practical lamp. Such a configurationentails an increase of approximately 50 percent in ratio ofcircumference or perimeter to area when averaged over the entire lengthof the envelope including the re-entrant portions and the interveninggaps, and is equivalent to flattening in a ratio of approximately 3to 1. Lamps so constructed have been operated and found to yieldapproximately 20 percent more light than equivalent conventional roundlamps under the same operating conditions. When the dimpled lamps wereoperated at a wattage to yield the same efliciency as the conventionalround tube lamps of the same perimeter, the lumen output of the dimpledlamps was approximately 60 percent higher.

Crenelated grooved lamp 31 shows in general operating characteristicssimilar to those of continuously grooved lamp 1, but modified inaccordance with the alternation of the groove sections on oppositesides. Both the upward and downward facing groove sections operate withenhanced brightness, so that the polar diagram of the luminous outputshows reinforced upward and downward components while the lateralcomponents are reduced. If the groove sections are of equal length onboth sides of the tube as shown in Fig. 3, the upward and downwardcomponents of the luminous output will be equally reinforced: by makingthe groove sections unequal in length, for instance by making the groovesections in the underside longer than in the top side, the downwardlight component may be reinforced more than the upward component.

(13) Implosion resistance The vastly improved implosion resistance ofvitreous tubes or envelopes, in accordance with my invention, isexplainable by two principles which may be applied either singly orjointly. The first principle is that the distribution of stressresulting from increasing the area of an envelope by means of are-entrant portion makes better use of the physical characteristics ofthin-walled envelopes made of vitreous materials, such as glass, whichhave low resistance to bending moments. This principle explains theincrease in implosion resistance afforded by the transversely re-entrantlongitudinal groove configurations of the envelope in Fig. 1. The secondprinciple is that the provision of double curvature, that is curvaturein two intersecting planes, in a vitreous body provides a definiteincrease in strength inasmuch as it eflfects a conversion of bendingmoments into compressive stresses by providing compressive or tensilesupport around the entire periphery of doubly curved areas, and isapplicable, along with the first principle, to the interrupted grooveconfiguration of the envelope of Fig. 2.

On the basis of the first principle, the improved implosion resistanceof the envelope with a longitudinally extending transversely re-entrantportion illustrated in Figsv 1 and 2 may be explained by analogy to aflat tube which has been bent into an inverted U-shape. By bending intoa U-shape, the strain in the convex outer wall 16 and in the concaveinner wall 17 reduces the strain in the convex joining edges 18, 18'(Figs. 5 and 6). Thus the stresses and the resulting strains aredistributed over four radii instead of over the two radii of the narrowedges in the plain flattened tube. Accordingly, the strains developedare less and, for a given thickness of envelope wall, the strength isgreater than in the case of the flat tube.

The second principle involved is that of double curvature and it appliesto the crenelated grooved lamp of Fig. 2. The situation may beanalogized to the conversion of radial forces due to the atmosphereacting on an evacuated sphere, into compressive stress in the walls ofthe sphere. Another way of viewing the matter is that a glass area ofdouble curvature may be considered to be supported in tension orcompression on all four sides instead of on two sides only as in thecase of a glass area of single curvature. According to formulae wellknown in the art of Strength of Materials, the stress developed in athin sheet by the application of a force at the center thereof variesinversely as the square of the thickness when the sheet is supported ontwo opposite sides only, and inversely as the cube of the thickness ofthe sheet when it is supported on all four sides. Thus, referring to theenvelope illustrated in Fig. 2, it can be seen that as the indentationsare made short enough, they approach the condition of being supportedall around instead of axially along the edges only. Thus, the conditionof a thin sheet supported on all four sides is approximated and thestress in the envelope walls is reduced accordingly.

14) Manufacture Several methods may be used to form the various enve='17 lope shapes of Figs. 1 and ,2. ,For small production, the mosteconomical is heating a suitable round envelope or tube, either beforeor after internal coating with the phosphor, to a plastic temperature,positioning the envelope into a suitable heated mold, and then clampingthe mold about the tube and allowing the whole to cool slowly below thestrain pointof the glass. Where the curves are relatively sharp, as inthe envelope of Fig. 2, aputf of a suitable gas may be blown into thetube to force the walls of the tube to conform closely throughout to thewalls of the mold. Where the envelope has been coated with phosphorprior to the molding operation, the gas blown into the tube should be ofa non-oxidizing kind in order to avoid spoiling the phosphor. For higherproduction rates the tubing may be shaped as it is drawn from the glassfurnace by using molds in the form of rotating wheels with grooved andsuitably shaped circumferences to form the tubing to the desired shape.For the envelope of Fig. 2 two such wheels with grooves of generallycircular cross section but having therein upstanding bosses alternatelyspaced in each wheel, may be placed at a point in the line of draw ofthe tubing where the glass is still plastic enough to form thedepressions or indentations.

While certain specific embodiments of the invention have beenillustrated and described in detail, various modifications will readilyoccur to those skilled in the art. It will be appreciated that whereasthese embodiments have been described with envelopes of glass, thefeatures of the invention relating to the inherent operatingcharacteristics of the re-entrant groove discharge device or lamp, suchas higher efiiciency, increased loading capacity, temperatureregulation, spreading or non-construction of the plasma and enhancedtransmission to the groove walls, are not necessarily or inherentlydependent upon use of glass or vitreous envelopes. In the case of a lampof course,-a radiation transmitting material must be used, however itneed only be pervious to the radiation desired to be emitted. For afluorescent lamp where the internal phosphor coating converts the 2537A. radiation to visible light, a material pervious to visible lightsuffices; for a germicidal lamp on the other hand, a material perviousto ultraviolet (2537 A.) is required; for a so-called black light lamp,it may be desirable to use material opaque to the visible spectrum butpervious to the desired ultraviolet; or again, as in sun-tanning lamps,it may be desired to use an envelope material pervious to the desirederythemal radiations along with a phosphor to eflect conversion of theresonance radiation to the erythemal (long-wave ultraviolet). Theappended claims are intended to cover any such modifications comingwithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge device comprising an elongated generallytubular envelope of vitreous material, a pair of electrodes sealed intoopposite ends thereof, and an ionizable medium within said envelopecomprising an inert starting gas at a low pressure and a small quantityof mercury, said envelope having transversely re-entrant portionsextending longitudinally substantially the length thereof, saidre-entrant portions providing a generally.

kidney-shaped cross section and being proportioned in depth andcurvature to afford an average ratio of circumference to area of crosssection for said envelope equivalent to that of a cylindrical envelopeflattened in a ratio in excess of two to one.

2. An electric discharge device comprising an elongated generallycylindrical thin-walled envelope of vitreous material, a pair ofelectrodes sealed into opposite ends thereof, and an ionizable mediumwithin said envelope comprising an inert starting gas at a low pressureand a small quantity of mercury, said envelope having a transverselyre-entrant groove extending continuously substantially the lengththereof to provide a generally I 18 kidney-shaped cross sectionaffording a ratio ,of circunn ference to area equivalent to that of acylindrical envelope flattened in a ratio in excess of 2 to 1 but havingan implosion resistance substantially greater than that ofv saidequivalentflattened' envelope.

3. An electric discharge .device comprising an elongated generallycylindrical envelope of vitreous material, a pair of electrodes sealedinto opposite ends thereof, and an ionizable medium within said envelopecomprising an inert starting gas at a low pressure and a small quantityof mercury, said envelope having longitudinally extending transverselyre-entrant spaced portions of length comparable to the maximum diameterof said envelope and providing a generally kidney-shaped cross section.I

4. A low-pressure positive column discharge lamp comprising an elongatedtubular thin-walled vitreous :envelope, a pair of electrodes sealed intoopposite ends thereof, and an ionizable medium within said envelopecomprising an inert starting gas, at a low pressure and a small quantityof mercury, said envelope having transversely re-entrant portionsextending longitudinally substantially the length thereof, saidre-entrant portions providing a generally kidneyrshaped cross sectionand being proportioned in depth and curvature to afford an average ratioof circumference to area of cross section for said envelope equivalentto that of a cylindrical envelope flattened ina ratio in excess of twoto one.

5. A low-pressure positive column discharge lamp comprising an elongatedtubular thin-walled vitreous envelope, a pair of electrodes sealed intoopposite ends thereof, and an inonizable medium within said envelopecomprising an inert starting gas at a low pressure and a small quantityof mercury, said envelope having longitudinally extending transverselyre-entrant portions of length comparable to the maximum diameter of saidenvelope and alternating on opposite sides thereof and providing agenerally kidney-shaped cross section.

6. A low pressure electric discharge device comprising an elongatedenvelope having electrodes sealed into opposite ends and containing anionizable medium wherein an electric discharge maintains a plasmacontaining atoms at an excited level which emit quanta of resonanceradiation as a result of transitions back to the normal ground levelwhich quanta eventually reach the plasma boundary by the process ofresonance radiation diffusion, said envelope defining a discharge spacehaving the general cross section of a sector of an annulus to allowsubstantial :diffusion of the plasma therethrough.

7. A low pressure electric discharge lamp comprising an elongatedradiation pervious envelope having electrodes sealed into opposite-endsand containing an ionizable medium wherein an electric dischargemaintains a plasma containing atoms at an excited level which emitquanta of resonance radiation as a result of transitions back to thenormal ground level which quanta eventually reach the plasma boundary bythe process of resonance radiation diffusion, said envelope defining adischarge space having the general cross section of a sector of anannulus wherein the rate of transmission of resonance radiation to theinner annular wall portion per unit area substantially exceeds that tothe outer annular wall portion.

8. A low pressure electric discharge lamp comprising an elongatedradiation pervious envelope having electrodes sealed into opposite endsand containing an ionizable medium wherein an electric dischargemaintains a plasma containing atoms at an excited level which emitquanta of resonance radiation as a result of transitions back 'to thenormal ground level which quanta eventually reach the plasma boundary bythe process of resonance radiation diffusion, said envelope having anouter wall of generally circular section and a longitudinally extendinggroove forming a re-entrant wall portion defining with the circular wallportion a discharge space having the general cross section of a sectorof an annu-. lus wherein the re-entrant wall portion receives resonanceradiation from the plasma per unit area at a rate substantially inexcess of that of the mean unit area of the envelope.

9. A lamp according to claim 8 having a vitreous envelope and whereinthe cross section of the discharge space has a ratio of annular breadthto maximum annular wall-towall spacing in excess of 2:l whereby toprovide a substantial increase in the ratio of perimeter to area over acircular sectioned envelope of the same perimeter in a lamp havingsubstantially greater implosion resistance than an equivalentlyflattened envelope and si multaneously realizing thereby a higher rateof transmission of resonance quanta per unit area of the re-entrantgroove wall portion than through the mean unit area of the envelope.

10. A low pressure positive column fluorescent lamp comprising anelongated vitreous envelope having electrodes sealed into opposite endsand containing an ionizable medium including an inert starting gas andmercury vapor wherein an electric discharge maintains a plasmacontaining mercury atoms at an excited level which emit quanta ofresonance radiation as a result of transitions back to the normal groundlevel which quanta eventually reach the plasma boundary by the processof resonance radiation diffusion, said envelope having an outer wall ofgenerally circular section and a longitudinally extending groove forminga re-entrant wall portion defining with the circular wall portion adischarge space having the general cross section of a sector of anannulus and wherein the re-entrant wall portion receives resonanceradiation from the plasma per unit area at a rate substantially inexcess of that of the mean unit area of the envelope, and a phosphorcoating on the internal surface of said envelope responsive to saidresonance radiation and achieving in said re-entrant Wall portion asubstantially higher brightness than over the remainder of the envelope.

11. A low pressure positive column lamp comprising an elongated vitreousenvelope having electrodes sealed into opposite ends and containing anionizable medium including an inert starting gas and mercury vapor wherein an electric discharge maintains a plasma containing mercury atoms atan excited level which emit quanta of 2537 A. resonance radiation as aresult of transitions back to the normal ground level which quantaeventually reach the plasma boundary by the process of resonanceradiation diffusion, said envelope having an outer wall of generallycircular section and a longitudinally extending groove forming are-entrant wall portion defining with the circular wall portion adischarge space having the general cross section of a sector of anannulus and wherein the re-entrant wall portion receives resonanceradiation from the plasma per unit area at a rate substantially inexcess of that of the mean unit area of the envelope, the degree ofequivalent flattening of said envelope as determined by the ratio ofannular breadth to maximum annular wall-to-wall spacing in the dischargespace being between the limits of 2:1 and 10:1, and the degree of taperin the wall-to-wall spacing in said annular discharge space being lessthan 50% from the center to the edges.

12. A fluorescent lamp according to claim 11 which includes a phosphorcoating on the internal surface of the envelope responsive to 2537 A.radiation and achievmg in said re-entrant wall portion a substantiallyhigher brightness than over the remainder of the envelope.

13. A low pressure positive column resonance radiation lamp comprisingan longated vitreous envelope havmg electrodes sealed into opposite endsand containing an ionizable medium including an inert starting gas at alow pressure and mercury vapor wherein an electric discharge maintains aplasma containing mercury atoms at an excited level which emit quanta of2537 A. resonance radiation as a result of transitions back to thenormal ground level which quanta eventually reach the plasma boundary bythe process of resonance radiation difiuslon, said envelope defining adischarge space having the general cross section of a sector of anannulus wherem the inner annular wall portion receives resonanceradiation from the plasma per unit area at a rate substantially inexcess of that of the mean unit area of the envelope and wherein wallloadings in the range of 0.05 to 0.08 watt per cm. result in optimummercury vapor pressure for generation of 2537 A. radiation. I

14. A low pressure positive column resonance radiation lamp comprisingan elongated vitreous envelope having electrodes sealed into oppositeends and containing an ionizaole medium including an inert startlng gasat a low pressure and mercury vapor wherein an electric dischargemaintains a plasma containing mercury atoms at an excited level whichemit quanta of 2537 A. resonance radiation as a result of transitionsback to the normal ground level which quanta eventually reach the plasmaboundary by the process of resonance radiation diflusion, said envelopehaving an outer wall of generally circular section and a longitudinallyextending groove forming a re-entrant wall portion defining with theC1!- cular wall portion a discharge space having the general crosssection of a sector of an annulus and wherein the re-entrant wallportion receives resonance radiation from the plasma per unit area at arate substantially in excess of that of the mean unit area of theenvelope, said lamp having a degree of equivalent flattening asdetermined by the ratio of annular breadth to maximum annularwall-to-wall spacing in the discharge space in the range of 2:1 to 10:1and being operable with wall loadings in the range of 0.05 to 0.08 wattper cm. for optimum mercury vapor pressure for generation of 2537 A.radiation.

15. A lamp according to claim 14 supporting linear loadings from 20 to40 watts per linear foot to eflect optimum mercury vapor pressure forgeneration of 2537 A. radiation.

16. A low pressure positive column fluorescent lamp comprising anelongated vitreous envelope having electrodes sealed into opposite endsand containing an ionizable medium including an inert starting gas at alow pressure and a small quantity of mercury wherein an electricdischarge maintains a plasma containing atoms at an excited level whichemit quanta of 2537 A. resonance radiation as a result of transitionsback to the normal ground level which quanta eventually reach. theplasma boundary by the process of resonance radiation diffusion, saidenvelope having an outer wall of generally circular section andalongitudinally extending groove forming a re-entrant wall portion in itsunderside which defines with the circular wall portion an invertedU-shaped discharge space having the general cross sec tion of a sectorof an annulus and wherein the groove portion receives resonanceradiation from the plasma per unit area at a rate substantially inexcess of that of the mean unit area coating on the internal surface ofsaid envelope responsive to said resonance radiation and achieving inthe groove a substantially higher brightness than over the remainder ofthe envelope resulting in a polar light distribution pattern providingan increase of approximately 5 0% in light output in the quadrant of thecross section which includes the groove.

17. A fluorescent lamp according to claim 16 having a degree ofequivalent flattening as determined by the ratio of annular breadth tomaximum annular wall-to wall spacing in the discharge space in the rangeof 2:1 to 10:1 and being operable with wall loadings in the range of0.05 to 0.08 watt per cm. and linear loadings from 20 to 40 watts perfoot to eflfect optimum mercury vapor pressure for generation of 2537 A.radiation in the range of 1 to 20 microns as determined by thetemperature at the lower ends of the legs of the inverted U- 21 shapedcross section, and a light generation efficiency not substantially lessthan 50 lumens per watt.

18. A fluorescent lamp according to claim 16 having a degree ofequivalent flattening as determined by the ratio of annular breadth tomaximum annular wall-towall spacing in the discharge space ofapproximately :1, and supporting a wall loading of approximately 0.07watt per cm. a linear loading of approximately 35 Watts per foot, withan average current density of approximately 0.2 ampere per cm?throughout the cross section of the discharge space to effect generationof 25 37 A. radiation at approximately optimum mercury vapor pressure inthe range of 1 to 20 microns as determined by the temperature at thelower ends of the legs of the inverted U-shaped cross section, and alight generation efficiency not substantially less than 50 lumens perwatt.

19. An evacuated electric device comprising an elongated vitreousenvelope of generally tubular form having a longitudinally extendingtraversely re-entrant groove portion defining a cross section of thegeneral shape of a sector of an annulus bounded by a convex outer wallof minimum curvature, a concave inner wall of greater curvature, convexedge walls of maximum curvature having a radius of curvature less thanthat of the concave inner wall to an extent not exceeding 50%, saidconvex edge walls being joined to the convex outer wall, and outwardlydiverging substantially straight wall sections inclined at an angle ofat least 15 to the medial plane of the groove joining the concave innerwall to the convex edge walls, whereby to realize a substantial increasein the ratio of perimeter to area along with maximum implosionresistance of the envelope.

20. A device according to claim 19 wherein said concave inner wall has aradius of curvature approximately one-third that of the convex outerwall.

21. A device according to claim 19 wherein said concave inner wall has aradius of curvature approximately one-third that of the convex outerwall and wherein the inclination of the outwardly diverging wallsections to the medial plane of the groove is approximately 27.

22. A low pressure positive column fluorescent lamp comprising anelongated vitreous envelope of generally tubular form having electrodessealed into opposite ends and containing an ionizable medium includingan inert starting gas at a low pressure and a small quantity of mercurywherein an electric discharge maintains a plasma containing atoms at anexcited level which emit quanta of 2537 A. resonance radiation as aresult of transitions back to the normal ground level which quantaeventually reach the plasma boundary by the process of resonanceradiation diffusion, said envelope having a longitudinally extendingtransversely re-entrant groove portion defining a cross section of thegeneral shape of a sector of an annulus bounded by a convex outer wallof minimum curvature, a concave inner wall of greater curvature, convexedge walls of maximum curvature having a radius of curvature less thanthat of the concave inner wall to an extent not exceeding 50%, saidconvex edge walls being joined to the convex outer wall, and outwardlydiverging substantially straight wall sections inclined at an angle ofat least 15 to the medial plane of the groove joining the concave innerwall to the convex edge walls, and a phosphor coating responsive to said2537 A. radiation on the internal surface of said envelope.

'23. A resonance radiation lamp comprising an elongated vitreousenvelope having electrodes sealed into opposite ends and containing anionizable medium wherein an electric discharge maintains a plasmacontaining atoms at an excited level which emit quanta of resonanceradiation as a result of transitions back to the normal ground levelwhich quanta evenutally reach the plasma boundary by the process ofresonance radiation difiusion, said envelope having an outer wall ofgenerally circular section and a plurality of longitudinally extendinggroove portions alternating on opposite sides of the envelope andforming re-entrant Wall portions defining with the circular wall sectiona discharge space having the general cross section of a sector of anannulus and wherein the re-entrant wall portions receive resonanceradiation from the plasma per unit area at a rate substantially inexcess of that of the mean unit area of the envelope.

24. A lamp according to claim 23 wherein the length of the re-entrantwall portions is not in excess of approximately 3 times the maximumdiameter of said envelope. 7

25. A lamp according to claim 23 wherein the ionizable medium includesan inert starting gas and mercury vapor producing 2537 A. resonanceradiation, and wherein the degree of equivalent flattening of theenvelope as determined by the ratio of annular breadth to maximumannular wall-to-wall spacing opposite the re-entrant wall portions liesin the range of 2:1 to 10: 1.

References Cited in the file of this patent UNITED STATES PATENTS2,135,480 Birdseye Nov. 8, 1938 2,190,009 Boucher Feb. 13, 19402,229,962 De Reamer Jan. 28, 1941 2,317,265 Foerste Apr. 20, 19432,687,486 Heine Aug. 24, 1954 2,714,682 Meister Aug. 2, 1955 FOREIGNPATENTS 861,799 France Nov. 4, 1940 123,425 Australia Dec. 1, 1944

